This invention relates generally to the servicing of high-voltage power lines and more particularly to a novel system for placing workmen in close proximity to a bundle of power lines to be repaired.
Electrical power is provided to the consumer over long distances by the utilization of high voltage lines which have the capacity of carrying voltage varying from 5 kilovolts (kv.) to 500 kv., and higher. Ultimately this high voltage is stepped-down for use by the consumer. In order to convey this voltage over long distances it is necessary and economical to utilize high voltage power lines.
The standard high voltage A.C. transmission line system will utilize three phases of conductors, and, if more than one conductor is used per phase, the group of conductors is called a bundle, supported generally parallel to one another, each carrying voltage 120.degree. out of phase to each other.
These high voltage lines are generally supported by structures varying from simple wooden poles with cross bars to extremely tall metal towers. As the demand for electricity over greater distances has increased, the trend has been to the higher capacity power lines of 240 HV and EHV 500 kv. and above which incorporate extremely tall lattice towers or tubular steel structures in excess of 75 feet and may extend up to 300 feet high for support. These power line support structures or towers generally utilize three types of insulating configurations for supporting the conductors consisting of individual porcelain covered discs which interconnect to form chains or strings of varying lengths. These insulator strings, as they are commonly referred to, are suspended from the towers either vertically, horizontally or a V-shape formed by two strings. These strings function to provide stand off air gap distances between the high voltage power lines and the tower structure which is grounded. This air gap distance is required to prevent the high power voltage line from arcing to the grounded support structure over the distance provided by the insulated strings referred to as insulator string flash-over. Arcing may be a relatively harmless discharge or transfer between two differing ungrounded potentials to equalize the voltage. If neither of the potentials are grounded there is no fault current. This ungrounded discharge between differing potentials will generally not occur unless they are very close together, usually a matter of inches, and typically up to 1.5 feet with extra high voltage lines of 500 kv and above. These types of discharges are generally not hazardous to personnel or structures since minimal current flow occurs.
The more serious and lethal discharge is the fault discharge between differing potentials, one of which is grounded. The fact that one potential is grounded creates a greater stress on the insulating medium, whether it be the insulating string or an air gap to convey the charge of the power line to the grounded object. In order to prevent this type of flash-over, a greater distance must be maintained between the differing potentials, as in this case the distance between the power line to any structure or object, whether it be a man, vehicle or the tower that is grounded.
Prior art discloses several wire cable, or conductor supported skips or worker support cages. An early U.S. Pat. No. 900,713 discloses an unmotorized cable way including a pair of baskets which alternately act by gravity to convey people or objects across an open span. An earlier U.S. Pat. No. 844,641, specifically discloses a manually-propelled lineman's chair for supporting a single workman onto a telephone line or the like for repairing existing lines or installing new parallel lines. This invention is limited to traversing from pole to pole with no attention in the invention to means for conveying the device beyond or around a pole. The worker therein is supported by a seat without any surrounding guards.
A later U.S. Pat. No. 3,702,124 also discloses a traversing manually or power propelled skip which is supported on two parallel power lines. The skip includes a caged basket which hangs directly under the power lines for safely supporting a workmen therein.
Likewise, U.S. Pat. No. 4,163,480 also discloses a line traversing skip which is supported by and traverses on at least one pair of power lines. Therein also the workmen supporting cage is hung directly below the power lines which support the device.
These last two above disclosures include means for moving these devices over any hardware connected to the lines, but do not include specific means for placing on or removing the devices from the supportive power lines. Typically, prior art discloses that the only means for lifting these cages or skips up onto the bundle of conductors is by ground-based crane or by block-and-tackle supported by the structure or conductors, the cage then hoisted up from beneath the conductors thereonto. Wheels or rollers that engage the conductors fold or swing out of the way until the cage is in position relative to the conductors, after which they are unfolded or swung back. Further, because the crane is grounded, this operation must be done only after power flow is interrupted.
This inventor is aware of two now commercially available cages for servicing power lines. These cages, or conductor carts as they are generally referred to in the catalog, are manufactured by Sherman & Reilly, Inc., of Chattanooga, Tenn. and are available in three models: manual, full power and single conductor. All of these cages place the workmen underneath or in between the transmission lines. Further, elaborate and expensive means are incorporated in both the manual and full power models for moving the support rollers and attached brackets out of the way of the power lines as the cages are crane-raised or block-and-tackle hoisted from beneath into position on the conductors.
The instant invention provides novel twin-workman-supportive baskets as part of the cage, which baskets hang, not directly below the support cables which may require repair, but laterally outward and slightly below the power lines in a unique generally inverted "U" cross-sectional shape to better facilitate repairs and to allow opposing workmen, at least one in each basket, to simultaneously work on adjacent repairs or to opposingly assist one another on specific repairs. Where only one workman is required, a dead-weight counter-balance may be placed in the other basket. From a point in space directly above the conductors, this cage may be lowered onto the bottom phase conductor or ground wire when other conductors are positioned directly above the objective conductor. This "U"-shape also facilitates lowering the cage directly onto the bundle of conductors without need for any disassembly or reassembly of the cage. This invention also includes at least one elongated insulated safety link as part of a support or lifting harness which allows the cage to be portably moved about, onto, and from power lines by helicopter or ground-based crane with workmen in the twin cages without fear of significant current transfer to the workmen or the helicopter or crane while working on energized power lines.